Within the framework of the production of natural gas (containing mainly methane) or liquefied natural gas, it is necessary to purify said natural gas, which originates from a deposit, by removing a certain number of contaminants, including primarily what are called “acid gases”, i.e. carbon dioxide (CO2), hydrogen sulphide (H2S), mercaptans (R—SH), carbonyl sulphide (COS) and carbon disulphide (CS2).
Carbon dioxide and hydrogen sulphide can represent a significant part of the gaseous mixture originating from a natural gas deposit, typically from 3 to 70% (in molar concentration). COS is present in smaller quantities, typically varying from 1 to 50 ppm by volume.
The contaminants which have to be removed include mercaptans, molecules of formula R—SH where R is an alkyl group. The total quantity of mercaptans in a gaseous mixture originating from a natural gas production site can represent a few hundred ppm by volume. The main two mercaptans concerned are methyl mercaptan and ethyl mercaptan, but other mercaptans (in particular molecules of type C3SH to C6SH) can also be present, generally at a lower concentration.
Numerous methods currently exist for deacidifying and removing mercaptans from natural gas (simultaneously or sequentially), using solvents capable of absorbing mercaptans and/or other acid gases chemically and/or physically (by dissolution).
Among the processes currently in use on an industrial scale, the so-called “Sulfinol” process involves eliminating the H2S, CO2, COS, CS2 gases and the natural gas mercaptans using a solvent constituted by a mixture of sulpholane, water and an amine (such as diisopropanolamine or methyl diethanolamine). Another example is the so-called “Selexol” process, which uses a solvent based on a dimethyl ether of polyethylene glycol.
Numerous other variants have been proposed, using alternative solvents. By way of example there can be mentioned solvents based on alkanolpyridine (U.S. Pat. No. 4,360,363).
However, there is still a real need to discover other solvents capable of effectively absorbing, preferably simultaneously, the mercaptans and other acid gases present in a gaseous mixture.
In particular there is a need to discover solvents making it possible to implement processes for the deacidification and demercaptanization of gaseous mixtures with a lower solvent flow rate compared with the state of the art (at a comparable gaseous mixture flow rate), and more generally at a lower cost compared with the state of the art.